SMT involves applying a solder-containing paste to a PCB, placing electrical and electronic components on appropriate places on the surface of the PCB, and passing the entire assembly through an infrared reflow oven that serves to melt the solder and permanently affix the components to the PCB. Older, through-hole methods required that holes be drilled and that each component be individually soldered in place. SMT has permitted the manufacture of smaller and denser layouts than were possible using through-hole techniques, and the resulting boards are generally cheaper to manufacture.
Because of their physical properties such as high mechanical properties and high heat defelection temperatures, flame-retarded, reinforced high-melting polyamides such as those based on terephthalic acid, adipic acid, and hexamethylenediamine or terephthalic acid, hexamethylenediamine, and 2-methyl-1,5-pentanediamine that have melting points greater than about 280° C. have been conventionally used for those electrical and electronic components for SMT application. The components are made from such polyamide composition by using a melt-processing method such as injection molding.
High melt flow (or low melt viscosity, as these terms are used interchangeably) is a very desirable characteristic of a melt-processable polymer resin composition, as it allows for greater ease of use in processes such as injection molding. A composition with higher melt flow or lower melt viscosity can be injection molded with greater ease compared to another resin that does not possess this characteristic. Such a composition has the capability of filling a mold to a much greater extent at lower injection pressures and temperatures and a greater capability to fill intricate mold designs with thin cross-sections. For a linear polymer there is generally a positive correlation between polymer molecular weight and melt viscosity.
It is also often desirable to add additional, often non-miscible, components such as glass reinforcing agents, or flame retardants to a polymer resin to achieve desired physical properties or flame retardancy. However, the presence of such components often leads to an increase in the melt viscosity of the resulting resin. Furthermore, these additional components are typically added using a melt blending process, and will preferably be sufficiently well dispersed in the polymer matrix to obtain optimal physical properties. The dispersal of the components during melt blending will often occur more efficiently when the polymer matrix has a high viscosity.
When the matrix polymer is a condensation polymer such as a polyamide, it is often possible to obtain a composition that has both well dispersed additives and a low melt viscosity by using high molecular weight matrix polymer in conjunction with a molecular-weight reducing additive in the melt blending process. In this method, the matrix polymer will have a sufficiently high melt viscosity to ensure an adequate dispersion of additives and the action of the molecular-weight reducing agent will result in a lower molecular weight matrix polymer. U.S. patent application publication 2003/0018135 discloses the use of aliphatic organic acids in the preparation of impact modified polyamide compositions that have both good melt flow and toughness. However, it has been discovered that the use of aliphatic acids disclosed in this publication can lead to rapid corrosion of the steel elements of the processing equipment used in the melt blending processes.
When the composition contains flame retardant, it can be used in SMT applications. However because of the absorption of water associated with these polymers, voids are formed therein when they are heated in ovens during processing. The resulting parts are consequently undesirably deformed.
In response to these concerns, high temperature polyamides incorporating flame-retardants have been used and are generally satisfactory for conventional applications. However, there is increasingly a demand for higher melt temperature solder materials that have more stringent property requirements than those of current polyamides in the industry.
According to the down sizing trend of electrical & electronics devises, the need of high melt flow is increasing for the resins used in the SMT applications. U.S. patent application publication 2006/0030693 discloses the use of terephthalic acid as an agent to increase melt flow of a high temperature polyamide composition. However, it has been discovered that the use of terephthalic acid disclosed in this publication can lead to deterioration of resistance against the void formation at SMT process.
Also, out-gassing from the polyamide composition at a molding process may cause problems such as mold deposit on surface of mold cavities.
There is a need for a high temperature flame-retarded, reinforced polyamide molding composition which is suitable to withstand the severe constraints associated with the manufacture of electrical or electronic components. A feature of the present invention is its advantageous improved melt flow and less out-gassing that also can withstand higher oven temperatures without the formation of voids and the deformation of those molded components or parts.